Distortion

The Sigma lens produces only a marginal amount of barrel distortion (~0.3%) which is not objectionable in field conditions.

Vignetting

The light fall-off is visible at f/2.8 (~0.7 f-stops) albeit comparatively moderate. Stopping down results only in a minor improvement but this amount of vignetting is no longer disturbing.

MTF (resolution)

The Sigma AF 30mm f/2.8 EX DN is capable of delivering an outstanding center performance straight from f/2.8 - it even broke the existing record for the Sony NEX 7 as of the time of this review. The lens is basically "diffraction-limited" in the image center so stopping down has no effect on the center performance anymore. The outer image regions reach a good level at f/2.8 and improve to very good (just) figures at f/4 to f/8. The overall performance diminishes from f/11 onward - this is a typical diffraction effect and no problem of the lens.

The centering quality of the tested sample was very good and there's only a very slight degree of field curvature.

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic Aberrations (CAs)

The Sigma lens produces a relatively high amount of CA (color shadows at harsh contrast transitions) with an average pixel width of around ~2px at the image borders. This aspect is certainly a weakness.